Motion of a substitutional impurity via a fast-diffusing intermediate species is discussed. An analytical solution is given for the case of low impurity concentration. For short times, few atoms migrate and the solution behaves exponentially. We observe this exponential signature in the diffusion of nanometer-scale B-doping profiles in Si. The migration frequency during oxidation-enhanced diffusion is consistent with diffusion-limited kickout of an interstitial-type B species. This is the first direct experimental evidence for dopant diffusion in Si via an intermediate species.

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The time evolution of B diffusion and electrical activation after ion implantation and annealing at 800 and 900 °C is studied using secondary-ion mass spectrometry and spreading-resistance profiling. The time evolution at 800 °C is observed in both crystalline and post-amorphized samples. Amorphized samples show near-normal concentration enhanced diffusion. Crystalline samples show anomalous transient diffusion, with a rapidly diffusing low-concentration region and a static peak region above a critical concentration Cenh=3.5×1018 cm−3. The peak region above Cenh is shown to be electrically inactive. The static, inactive B is released over a period of many hours, compared with the transient diffusion enhancement which relaxes to near-normal within 30 min. The time evolution of B diffusion at 900 °C is observed as a function of implantation dose. A critical concentration for transient diffusion, Cenh=8×1018 cm−2, independent of dose, is observed at this temperature. The transient diffusion enhancement in the diffusing part of the B profile increases with dose, up to a dose of ∼5×1014 cm−3, and saturates at higher doses. A comparison with published data shows that Cenh∼ni within a factor 2 over the temperature range 550–900 °C. We interpret our observations in terms of a nonequilibrium point-defect model of diffusion and intermediate defect formation.